I did some math today on the CPAP and battery power requirements. Thought I'd share it here in case it will help others. Your CPAP will have power markings on it, either watts, or Volts and Amps or mA (milliamps). In general terms, you need to know how many Wh or Watt hours your machine uses. I'll run thru mine and the math so you can see how to figure out your needs.
My machine is marked 12V and 5 Amp. To get Watts, this is the equation- Volts x Amps = Watts, or for my CPAP, 12V x 5A = 60 Watts. Watt Hours, (Wh) is how many watts are used over the time, or the hours you want to run the device. So 60W x 8H (hours) = 480 Wh. This is the power you would need to run my 60 Watt CPAP for 8 hours.
Great, so now what about the battery needed? Batteries are generally measured in Ah, (amp hours) or mAh, (milliamp hours). So how many Ah are needed to run for 480 Wh?
That equation is: Wh/12 = Ah OR Wh x 1000/12 = mAh
So, 480W/12 = 40 Ah OR 480 x 1000/12 = 40,000 mAh
So to run my 60W CPAP for 8 hours I would need a 40,000 mAh battery. That's a big/expensive battery.
When comparing batteries, it's simplest to know your Wh for your device and plug the mAh of the battery into this formula to get Wh. mAh x 12/1000 = Wh.
So the AntiGravity Micro Start model XP-10
http://antigravitybatteries.com/ is rated for 18,000 mAh. 18,000 x 12/1000 = 216 Wh. 216/60 (the watts my device uses per hour) = 3.6. So my 60 watt CPAP would only run for 3.6 hours on the XP-10, which has a MSRP of $209 measures 9" x 3" x 1.25" and weighs 18 ounces. The possibly soon to be out MAX 36 from AntiGravity is spec'd for 36,000 mAh and would last 432 Wh or run the CPAP for 7.2 hours, enough for a decent night's sleep.
Keep in mind that the rating of the CPAP might not be absolute in how much power it draws under real life conditions, AND if you use a humidifier, you're going to need to calculate that part of the unit's draw and add it to your total Wh needs to calculate battery Wh needs.
This 12V 40Ah deep cycle battery only costs $44.39, is a little large and weighs 23 lbs, but it an example of a lower cost deep cycle battery that would do the job.
http://www.apexbattery.com/universal-battery-u1-40-12v-40ah-battery.html?utm_source=googlepepla&utm_medium=adwords&id=124409536626&gclid=Cj0KEQiAu9q2BRDq3MDbvOL1yaYBEiQAD6qoBtaItTQmXhT9iBjnb1SDAtxiDEinJnbLUEoPOg6jtjAaAqZ08P8HAQ
Hope that helps someone. It helped me figure out what I would really need for battery power for my CPAP and gave me a better idea of what I might want to do for a home back up too.